In the motor vehicle industry it is well known to maintain a voltage which is supplied to the on-board electrical network by an alternator of the vehicle at a predetermined set value, independently of the speed of rotation of the engine or the electrical consumption of the equipment, by means of a regulation device known as a “regulator”.
This regulator, which is generally incorporated in the alternator, controls an excitation current which is supplied by a battery and circulates in an excitation winding of the alternator.
At present, motor vehicle parts manufacturers have developed very high-performance alternators by implementing electronic power systems controlled by circuits which use digital techniques, based in particular on the use of microprocessors or microcontrollers.
These techniques make possible distinctly better stabilisation of the voltage of the on-board network than the previous bimetals, in response to the activation of substantial electric charges in the vehicle.
However, it can be conceived that an increased requirement for excitation current by the regulation system must not lead to a rapid increase in the torque taken out by the alternator from the thermal engine, which would be liable to stall the thermal engine, particularly when it is idling or is cold, when the vehicle is started.
It is thus known to limit the resistant torque taken out by an alternator from the thermal engine by means of a progressive charging function known as “LRC” (an acronym for “Load Response Control”). Control of the intensity of the excitation current is generally obtained by variation of the duty cycle of an excitation signal of the PWM (“Pulse Width Modulation”) type which controls a power switch of the excitation circuit.
In a known manner, and further to detection of a charging requirement, the LRC function authorises only progressive increases of the duty cycle of the excitation signal from the initial value to the value determined by the control loop, by increasing this initial value by using a so-called “progressive charge return” intermediate signal.
Also in a known manner, the excitation signal which actually controls the excitation circuit is derived from the multiplexing of the excitation requirement signal generated by the control loop and the excitation signal produced by the LRC function, under the control of the LRC function.
However, the above-described type of LRC function is simply a systematic open loop action which does not really interact with modification of the on-board voltage or the speed of rotation of the engine.
In these conditions, the electronic control unit of the engine can have problems with stabilising the engine speed during idling.